As I said in another post, Mormon boys are going to continue to go on two year missions. Those that choose to go on missions will be two years older than their peers. If you feel that taking a two year break from training is advantageous to your ultimate running goals, there are no rules precluding you from doing so. In fact, it is within NCAA rules to take two years between high school and college and run 100 miles a week as long as you don't race after the first year removed from high school and still have 4 years of eligibility when you start college. Where some schools might be in violation of the age rule is that after their first year post high school, if an athlete has competed then their 5 year clock should have started.
The advantage that BYU has that others don't is that the LDS church has the infrastructure to support this two year hiatus from training and school. Culturally the LDS religion encourages 18 year olds to take the two years off for missionary service and then sends them around the world to places that have an organization in place to manage their service, keep them safe and housed. These kids have either saved money to pay for their missions or in most cases are supported financially by their families. They are not paid for their 60 hours plus of proselyting per week, but rather pay from their own account 400.00 per month for the privilege.
BYU (Eyestone) has also figured out how to manage his roster so he can get these guys off their missions, red-shirt them at least one or two semesters until they are back in shape and then put them back into action. The result is that at any given time he has 20% of his roster redshirting as they get back in shape.
I have a lot easier time getting behind a 24 year old American senior who has taken two years away from school on his own dime, then for an older ringer brought in from a foreign country. Especially when these Americans go on to represent the US so well in international competition like Rooks, Mantz and Young.
Also, not every athlete at BYU chooses to serve a mission. Those that do will be only 2 years older. I have seen posts indicating ages of 27 etc. This is hyperbole. When the COVID extra year sorts itself out, there would be no reason that a BYU athlete would be more than two years older than any other "traditional" NCAA athlete.
If the two year returned missionary is such an advantage, other schools should invest in the opportunity to reap the benefit. The truth, besides the obvious cultural draw to BYU for returned missionaries, is that most NCAA coaches don't have the patience to wait two years for the prized LDS high schooler to return from a mission and then take an extra year to get the out of shape returned missionary back into racing condition. Until other schools decide to take that gamble, this will be the BYU advantage.